Check the incidence on the wings. (The bottom wing should be 0 (zero) and the top wing should be -1 (negative one) degrees. ) They are calling for them to both be parallel to the elevator (page 14), but it sounds like you may need to adjust that a little. . Also make sure you are CG'ing it on the TOP WING

Per the manual:
"The recommended Center of Gravity (CG) location for the Pitts
Model 12 15e ARF is 3 3/4– 4 1/8-inch (95–105mm) back from
the leading edge of the top wing"